Cathode-ray beam deflection circuit



May 23, 1950 w. MlLWlTT ETIAL 2,508,926

CATP IODE-RAY BEAM DEFLECTION CIRCUIT Filed Aug. 2a, 194*:

EE f0 CHW'ER 727E 50 EARL H. SCHOENFELD WILLIAM MWITT MMQ AT ORNEY Patented May 23, 1950 UNlTED ATENT CATHODE-RAY BEAM DEFLECTION CIRCUIT of Delaware Application August 28, 1947, Serial No. 771,048

3 Claims. (Cl. 31526) The present invention relates to cathode ray beam deflection circuits of the electrostatic type, and more particularly relates to means for overcoming certain non-linearities of scan which may result from the presence of inductive loading in the output circuit of the deflection generators.

In cathode ray beam deflection circuits of the above nature the two horizontal, or line, deflection plates of a cathode ray tube are frequently supplied with balanced potentials in order to prevent a de-focussing of the scanning beam. The means for obtaining such potentials may include a pair of power output tubes arranged in pushpull relation, and, especially in cases where the supply voltage is of relatively low value, a centertapped choke coil is frequently employed the ends of which are respectively connected to the anodes of the power tubes. In such a circuit arrangement the effective load may be considered to include the choke coil in shunt with the capacitances of the power output tubes, the cathode ray beam deflection plates, and the wiring, as well as the distributed coil capacity itself.

If the impedance of this load is high relative to the internal resistance of the power tubes over a band of frequencies from the fundamental line frequency to approximately its tenth harmonic, there will be negligible phase and amplitude distortion in the sine wave components making up the sawtooth deflecting wave. This, however, requires that the choke coil have considerable inductance and a suiiiciently high Q at the fundamental line frequency. It also requires that the shunting capacitance be relatively small at th tenth harmonic of the line frequency.

When a choke coil having the above characteristics is employed, a series of vertical bars, or striations, frequently appear in the reproduced image. The bars are most noticeable near one side of the image raster and decrease in intensity in the direction of scanning. It has been found that these bars are caused by variations in the speed of the electron scanning beam as it moves across the face of the cathode ray tube, and that these variations in turn result from damped oscillations, or transients, in the horizontal deflecting wave. They may have a frequency, for example, of about 300 kilocycles per second.

' If the choke coil is considered to be made up of distributed inductance and capacitance, then it may be regarded as a virtual (or artificial) transmission line composed of a number of L-C (inductance and capacity) units connected in series, eachunit being composed ofone turn of wire together with the distributed capacitance of this turn to ground. As such, it will be appreciated that the appearance of a sharp voltage pulse at each end of the choke at the beginning of retrace time will cause a voltage wave to travel from each end of the choke toward the tapped center.

At this point, the wave encounters an A. C. (alternating current) short circuit imposed by the supply voltage source.

This short circuit produces a reflection of each voltage wave front back toward the plate of its respective power tube. However, since the tube impedance does not properly terminate the line, another reflection occurs, and this process continues until the reflected wave is damped to a negligible level. By thistime, however, the oathode ray scanning beam has been deflected a considerable distance across the image raster area.

The vertical bars, or striations, which are produced in the above manner may be substantially reduced by the expedient of terminating the choke coil transmission line with a resistance the value of which is approximately equal to the characteristic or surge impedance of the line. This resistance is preferably connected between the center tap of the choke coil and the positive terminal of the supply voltage source, this positive voltage terminal being in effect the termination of each of the two transmission lines formed by the two halves of the choke coil winding. Since the surge impedance of the lines in many cases is best determined experimentally, a variable resistor may be employed at such point which has a sumciently wide range of values to include the one required.

One object of the present invention, therefore, is to provide an improved type of horizontal, or line, deflection system for use with electrostatically-defiected image-reproducing cathode ray tubes.

Another object of the invention is to provide an improved cathode ray beam deflection system of the electrostatic type, in which means are included for overcoming the vertical bars or striations which are produced in the image raster due to internal voltage reflections in the choke-loaded push-pull amplifier.

A further object of the invention is to provide a system of balanced, or push-pull, electrostatic deflection for cathode ray tubes, in which a center-tapped choke coil in the output circuit of the push-pull amplifier is considered to be a virtual transmission line which is terminated by a resistor the value of which is approximately equal to the characteristic or surge impedance of the line.

' Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawing, in which:

Fig. 1 is a circuit diagram of a horizontal cathode ray beam deflection circuit of the electrostatic type, employing a center-tapped choke coil in the output circuit of a push-pull amplifier;

Fig. 2 illustrates a portion of Fig. 1 redrawn in such a manner as to'represent the choke coil as an artificial transmission line having distributed capacity to ground between each turn of the coil and Fig. 3 is a modification of 'Fig. *2, in which a resistor is inserted between .theeenten-tap of the choke coil and one terminal of the source of potential. r

Referring now to the drawing, there is shown an image-reproducing cathode ray tube generally indicated by the reference numeral "10. Tube 4 0 is an electrostatically-defiected cathode ray tube of any suitable type which includes a pair of 9ri 9r1ta gr l n defl c i p at s i2 and J4- Tilbe 1H w l a so 1 u d st d t i ud th usualelectrongun t5 Ior developing anddirecting cream 9 .el s n owa d he um se q tar e ne sh n W th nt b l The horizontal, or line; deflection plates I 2, a e mended with b l n fl ct n po e tials of substantially saw-tooth waveform from a lin q fles eneratc w ch in l d s a Pa of electron discharge devices I 6, l8. These electron discharge devices 16, {3 may comprise the W9 ,pQt of a t in t a (su h as'i 5m as Sh w r the wie fi -he anode 20 of the electron discharge device It i p npe t itp iho t onta fl in pl M by means of a conductor 22. Similarly, the;

anodez of the electron discharge device -3 is connected to the 'l lorizontal deflecting plate 12 by mean of q i li te it- In e e t at th potentials applied to the horizontal deflecting plates 12} Hi be-balanced to the control electrode fll of the electron discharge device 16 is supplied with acontrol voltage the wayeform'of which is'fapproxiinately that of a rising saw -tooth, as indicated in the drawing by the reference num mes. The control electro'de32 of the electron discharge device {8, on the other hand, is supplied with a control voltage the'waveformof which'is substantially that of a-failing saw-tooth, asihdicated in the drawing by the reference numeral 34. The control voltage waves 39 and 34', of course, are substantially -l80 out-of phase with one another, as is the usual procedure in circuits ofthetype described.

The anode 29 of the electron discharge device 7 lB-islconnectedtothe positive terminal of a sourceof potential-36 through one portion 3-1 of a choke coil -38,--the latter being provided with a centertap 39 in order to permit such connection. Correspondingly, the anode 24 of the electron'discharge device -i8-is Connectedtothe positive terminal of the supply voltage source 36 through the remaining portion 49 of the choke coil 38. The negative terminal of the supply voltage source-36 isgrounded as shown, asis also each cathodeof thetwin triode-ifi lti. 7

In considering the operation ofthe circuit of Fig. :1, the efiective load of'the electron discharge tubes 16, 18 may be taken to include thechoke coil '38 in shunt with the capacitances of the tubes L6, :18, the "cathode ray beam deflection plates l2, :l4, and the wiring capacity. The distributedcapacity of the choke coil 3% is also a factor.

When th m edanc rcf this l ad is h r lativezta-the interna resistancecf theiel etr n .d s-

4 charge tubes l6, I8 over a, band of frequencies from the fundamental line-scanning frequency to approximately its tenth harmonic, there will be negligible phase and amplitude distortion in 5 the sine wave components making up the sawtooth defiecting wave applied to the deflecting plate 4. 1. Thi ust r .5 t e .v if the choke coil .35 has considerable iinductanc 'e and a relatively high Q at theiundarnental line-scanning frequency. It also necessitates that the reactances of the shunting capacitances be relatively :low at the tenth harmonic of the linesca i ee .i s ucncy- ,o oke coil having the above charac I y causes a series of vertical bars, or ctratigir s, 3:0 appear in the reproduced image, these bars being more noticeable near one smear the image raster and decreasing in intensi ty in th e direction of scanning. They may have a frequency, 'for example, of about 300 kilocycles c ame p ge can be trac a t pret nse damped oscillations ,or t ra siente, n the oril' ga uc iiiii'i' iii j b lne wire tqge he wi h th acce tances 6f h e of th hq s .9 re fa nte he c ally hyithefcondensersyq 2 (shown dotted lines) L wil lr vfi b pv ec' te tha the j p sa 'arice of sha rqlta e pulse a h er l i nal' t 3 1 9 41 9 t e mor ge in reapin wane t e wil c use l e Wave weav rp'ritea h ii efii i 11 were e t nn d-c ter P9 91 39 A; th fpq gtth .vq t Wfa l wbunr aisfa e-.c'-. m it i tu tfit imr9 d. v 'fu1 1- H13 i l agesqii e gifi' This short circuit 1L8 pro v ea h zvq a wa ef octt tk tters th a -b it resp ve lzgw'ehtubi .L QQ l8- fi w vrgs nqe ncec ltheltube 1.5 audi v i i a l .d g e Laws the ima e T e. h9r .bi a l iisctsgf hs fle r n al r am. d .b t ex:

it a wi l t eatmetor 56 the yalue of which e h w -E18 preiera lyconnectec.b tween edance f th line. st

, .-ee; 91 5 ina .hei ns fg ttc at n o 3 c the W9 ft ansmiss r. .ina i th s k ss ion line yvith its characteristic edance. the line becomes infinitely .7 r a u t ei i h 9 gc rse vrecu t -i th ze i natio a ,r et 0f the everticgl bars otherwise present in he re n ucedimase.

s n etth .v lu .of resist rrfil in an cases i bestdete im ned v.iexperinr entally, :it .may, if -,de-

ely equa to;thelgharacteristic or tired, be.a ivisable to emp o a va able which has a suiiiciently wide range of values to include the one required.

In cases where the values of the remaining circuit components are known, such, for example, as in the mass production of television receivers, it is possible to incorporate the separate resistor '50 as an integral part of the choke coil 38 by saw-tooth voltages are supplied by a pair of power tubes connected in push-pull relationship, a center-tapped choke coil arranged so that one portion lies in the output circuit of one of said power tubes and the other portion thereof lies in the output circuit of the other of said power tubes, connections between the ends of said choke coil, respectively, and said pair of deflecting plates, a terminal for connecting the output circuit of each of said power tubes to a voltage source, and a terminating resistor connected between the said terminal and the center tap of said choke coil, said terminating resistor having a value approximately equal to the surge impedance of said choke coil when the latter is considered as a virtual transmission line.

2. A cathode ray beam deflection circuit according to claim 1, in which said terminating resistor is adjustable to determine the value thereof which is equal to the surge impedance of said choke coil.

3. In a cathode ray beam deflection circuit of the type in which two power output tubes arranged in push-pull relationship are adapted to supply balanced cyclically varying voltages of the two line deflection plates, respectively, of an electrostatically deflected cathode ray tube, and in which a tapped choke coil is employed in such a manner that one portion of said choke coil lies in the output circuit of one of the said power output tubes and the remaining portion of said choke coil lies in the output circuit of the other of said power output tubes, the tap on said choke coil being normally connected to the positive terminal of a D.-C. potential source, the improvement which comprises a terminating resistor having a value approximately equal to the surge impedance of the said choke coil, when the latter is considered as a virtual transmission line, connected between the tap on said choke coil and the positive terminal of the said D.-C. potential source, whereby reflections of the said cyclically varying voltages generated by the saidpower output tubes are prevented from occurring within the said choke coil.

WILLIAM MILWITT. EARL H. SCHOENFELD.

REFERENCE S CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 2,077,574 Maloii' Apr. 20, 1937 2,086,926 Stocker July 13, 1937 2,101,520 Tolson et al Dec. 7, 1937 2,149,077 Vance Feb. 28, 1939 

